Ustiloxins and Ustilaginoidins in the Sclerotia Generated from Rice False Smut Balls and Their Contents

Rice false smut (RFS) caused by the ascomycete Ustilaginoidea virens (teleomorph: Villosiclava virens) is a destructive fungal disease all over the world. The RFS balls are ball-like colonies transformed from individual grains through the infection of U. virens. The sclerotia, which are dormant structures, typically generate from the late-stage RFS balls, and produce ascospores that play a dominant role in the life cycle of the pathogen. U. virens can produce mycotoxins, mainly including ustiloxins and ustilaginoidins, that are toxic to rice plants and animals and pose a serious threat to their health. Though ustiloxins and ustilaginoidins have been isolated from the RFS balls, their distribution and contents in the sclerotia remain unclear. In this study, a systematic content analysis of main mycotoxins was conducted on the sclerotia and other parts of the late-stage RFS balls. Ustiloxins were predominantly found in the sclerotia and middle layer, whereas ustilagnoidins mainly accumulated in the outer and middle layers and rarely accumulated in the sclerotia and inner layer of RFS balls. The findings were further supported by transcriptome and RT-qPCR analysis data. The different accumulation and distribution of these two kinds of mycotoxins in the sclerotia and other parts of the RFS balls may be related to their specific physiological functions and deserve further investigation.